The techniques for the synthesis of lactide from lactic acid has been developed for several decades and the synthesis method thereof has been described in numerous patents, such as U.S. Pat. Nos. 5,053,522 A, 5,247,058 A, 5,357,035 A, 6,005,067 A, 6,277,951 B1, 6,326,458 B1, 5,274,127 A, US 20050222379 A1, US 20120302724 A1, US 20110155557 A1, CN 1951933 A, CN 1594313 A, CN 1488628 A, and WO 2010105143 A2. At the present stage, methods mainly used in industrial productions relate to heating at high temperatures and using tin-based chemicals as catalysts (such as tin (II) 2-ethylhexanoate and tin chloride) in a vacuum environment. In order to avoid the oxidation of the reactants under high temperature conditions (≥180° C.), the chemical reaction is usually carried out in an inert gas atmosphere (e.g., nitrogen).
The synthesis process involves two reactions as below:
1) Oligomerization: the lactic acid monomer undergoes a dehydration polymerization reaction to form an oligomer/prepolymer.

2) Depolymerization: the oligomer/prepolymer is depolymerized into cyclic dimer lactide.

In the depolymerization process, the reaction conditions of a high temperature and a low pressure can cause the oligomer to partially depolymerize and at the same time, causes the remaining oligomer to further polymerize into a polymer with a higher molecular weight. Conventional tin-based catalysts are more conducive to polymerization rather than depolymerization, resulting in increased difficulty in the synthesis and separation of lactide. Therefore, the production yield of lactide synthesis of a conventional method is usually merely 50-70%. For example, U.S. Pat. No. 5,053,522 A describes a method for the synthesis of lactide with a tin-based catalyst, in which a high temperature of 200-260° C. is required to separate the lactide product, and the production yield is 69%. In a single batch production in U.S. Pat. No. 5,274,127, lactide production yield is 56.8%. In addition, the low catalytic efficiency of other metal compounds for polymerization and degradation leads to a decrease in the production yield of lactide (<70%). For example, zinc oxide particles are applied in US Patent US 201203027 A1, and the lactide product is obtained by distillation separation at a high temperature of 230-240° C., with the production yield obtained being lower than 72%. Therefore, traditional lactide synthesis methods have a low production yield and need to be further optimized and improved.